on VFD supply by diesel gen set why to multiply by two the size of gen 4

[uhpo]

I was talking the other day with a friend of me and tell me that when you use a genset for starting in example some motors with VFD, you´ll need to overdimensioning the size of generator, this means in example if you are with 1,7 MW pf 0,8, this means 2,12 MVA, if you use VFD you would need about 4 MVA generator to drive the load, and not to have overheating on the generator, but i´m still don´t understand this point... could somebody explain it to me... cause my friend just tell me this is what the guys that selling the genset says....maybe it´s right we are not experienced on this VFD
thanks and sorry for my special english.. i hope you´ll understand

 

[uhpo]

yes. i´m still confuse, i read a lot of times mike post and i think i agree and it´s close to what i read and people tell me , but on the other hand i´m electrical guy and i understand and i like to understand why mike says that when the point is that the harmonics content didn´t affect too much, or maybe real VFD didn´t have such advantages and we still got a high component of harmonics.

 

[LionelHutz]

That's what I posted Gunner. You have to oversize the genset to be able to soft-start the motor.

Posting that you don't have to oversize the genset with a soft-starter since the soft-starter does not produce any full-speed harmonics is doing everyone reading a dis-service. The running harmonics are not the reason to oversize when using a soft-starter.

 

[Skogsgurra]

My 'No' was an answer to your 'does the fact that the genset doesn't need to be oversized due to soft-starter harmonics matter when the genset needs to be oversized so you can start the motor?'

That means that I agreed. Nothing else.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[catserveng]

I have found, at least in the size units I mostly deal with, this is a VERY hard discussion to have in general terms. So many factors affect how the system will perform dynamically, what does it really do when it starts and runs. You're getting lots of great general advice, but to do this properly the devil is in the details.

I got involved with my former employer and a customer installing two generators and two large sewage pumps, the system was a near exact copy of a system installed about 5 years ago. Same pumps, similar motors (same nameplate ratings and type), same VFD's (by model number but with some "improvements"), same physical layout of plant with very similar load conditions, and the same size (ekW and kVA rated) generators. First system worked flawlessly for years, after a few commissioning issues that were quickly resolved. The new system fell on it's rear end. The differences initally appeared to be minor but with fairly major consequences.

The old VFD's had a single fairly wide voltage and frequency tolerence, the new units had an on and off grid selection, that defaulted to on grid. This was found early and made some improvement but didn't resolve problems.

The main problem was the differences between the "new" and "old" gensets. Both were rated as standby's for 1000ekW, both rated for a 0.8 lagging power factor. As Bill pointed out above, standbys normally have no overload rating, but the pre-EPA rated engine had a bit more reserve and response capability than the new EPA tier 2 engine did, a fact of life nowadays in the US. So there was a difference in the engine response capability that was actually pretty small, but had significant impact.

There was a significant difference in the tail ends, the old 1MW tail ends were bigger, heavier, more iron and copper. The new and improved tail ends were smaller, lighter, cheaper, and "more efficient". Looking at the data was hard to see a possible difference, in reality the ability to deal with the real loads imposed by the VFDs and motors was quite different.

The excitation and AVR's were also different, the old dog used big diodes, and older design, but very robust AVR. The new generator had smaller diodes, a less capable AVR power source, and a digital AVR with factory default settings.

After a lot of data taking and tweaking, adjusting, and holding our collective breaths, we could get the system to work most of the time. To make it work reliably the generators will have to be retrofitted with larger tail ends and possibly a horsepower uprate, if the air board folks will buy off on that.

The reason for this long winded story (that is still not over with, will probably go to court) is to try and describe many of the factors that can be involved with trying to closely size a generator to a VFD/motor. It can be done, but I don't think even with all the great minds available to you here, that you can the info you need to make this work as you need from a forum.

What you can get from the advice presented by all who contribute is an idea of the skills needed by the engineer and equipment suppliers you select to help make your specific system function as needed.

Hope that helps, Mike L.

 

[waross]

I have spec'ed and installed quite a few generators. In this instance I would be guided by catserveng's comments.
Mike; Is there any way you can increase the fuel by changing injectors or the injector pump or are you combustion air limited and already blowing black smoke?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

Bill,

In this particular case our limitation is the nameplate rating of the unit and the air pollution permit. For many of the standby ratings in the market today, especially with EPA approved ratings, it is VERY hard to change an engines horsepower. This is an issue for CAT and many others. I'm afraid the days of being able to tweak a bit more out of an engine are near an end. The emissions folks don't want you to do it, and the folks selling the iron are selling HP, you want more, pay more.

I'm also seeing the same issue with tail ends, we used to have lots of extra capacity in many of the frame sizes, and we'd see full load field amps at about 1/2 the rated maximum excitation system rating. But the value engineering employed by many suppliers basically gives you exactly what you paid for and nothing more. Makes it tough when you're used to a bit of extra cushion that may have not been advertised but was available in the iron none the less.

Guess this is evolution, frankly I miss large, slow engines with huge tailends that ran for 100,000 hours with minimal problems. When I started with CAT we supplied lots of 399 and 398 engines with SRSE and SRCR generators for mostly the oil drilling market. They were beasts, nearly indestructible, ran for years, took lots of abuse. Electrically very tolerant of harmonics, noise, spikes, surges and abuse. But they had lots of iron and copper for their size, and the AVR's needed a small crane to handle. If you could get the customer to keep the slip rings in good shape, they performed well.

Of course the new stuff keeps me busy, so I guess I can't complain.

Mike L.

 

[bentov]

Interesting discussion. For a given combination of motor and generator then, would a soft starter give the best chance of success when the generator is closely sized (i.e. vs Autotransformer or Part Winding methods, for example)? I would've thought so, because of better control of accelerating current over time, but sounds like there's more to it than that . . .